Diesel engines are the most popular type of compression ignition engines. Diesel engines introduce fuel directly into the combustion chamber. Diesel engines are very efficient because they provide high compression ratios without knocking, which is the premature detonation of the fuel mixture inside the combustion chamber. Because diesel engines introduce fuel directly into the combustion chamber, the fuel injection pressure must be greater than the pressure inside the combustion chamber. For liquid fuels such as diesel, the pressure must be significantly higher so that the fuel is atomized for efficient combustion.
Diesel engines are favored by industry because of their excellent combination of power, performance, efficiency and reliability. For example, diesel engines are generally much less expensive to operate compared to gasoline fueled, spark-ignited auxiliary engines, especially in commercial applications where large quantities of fuel are used. However, one disadvantage of diesel engines is pollution, such as particulate matter (soot) and NOx gases, which are subject to increasingly stringent regulations that require NOx emissions to be progressively reduced over time. To comply with these increasingly stringent regulations, engine manufacturers are developing catalytic converters and other after-treatment devices to remove NOx and other pollutants from diesel exhaust streams.
Improvements to diesel fuels are also being introduced to reduce the amount of sulfur in diesel fuel, to prevent sulfur from de-activating the catalysts of catalytic converters and to reduce air pollution. Research is also being conducted to improve combustion efficiency to reduce engine emissions, for example by making refinements to engine control strategies. However, most of these approaches add to the capital cost of the engine and/or the operating costs.
Other recent developments have been directed to substituting some of the diesel fuel with cleaner burning gaseous fuels such as, for example, natural gas, pure methane, butane, propane, hydrogen, and blends thereof. Since gaseous fuels typically do not auto-ignite at the same temperature and pressure as diesel fuel, a small amount of pilot diesel fuel may be introduced into the combustion chamber to auto-ignite and trigger the ignition of the gaseous fuel. Another approach for combusting gaseous fuel in a diesel engine involves introducing the gaseous fuel into the engine's intake air manifold at relatively low pressures. However, this approach has been unable to match the performance and efficiency of currently available diesel engines, particularly at high gas: diesel ratios. Thus, the simultaneous delivery of both diesel fuel and gaseous fuel to the combustion chambers, with the diesel acting as a pilot fuel, may be desirable.
One problem associated with delivering a gaseous fuel and a liquid diesel fuel for injection directly into the combustion chambers of an internal combustion engine is the need to maintain a well controlled pressure differential between the gas and diesel fuel rails. Because the response time of high pressure direct injection (HPDI) fuel systems can vary, it may be necessary to vent some gaseous fuel prior to combustion to maintain the required pressure difference between the two fuel rails. The gaseous fuel may be vented to the atmosphere, which wastes the gaseous fuel. U.S. Pat. No. 7,373,931 discloses a method of delivering both diesel and a gaseous fuel to a combustion chamber using a common drain rail connected to drain passages of both the gaseous fuel injection valve and the diesel fuel injection valve. The common drain rail leads to a liquid fuel storage vessel, which is equipped with a venting device to vent the gaseous fuel to the atmosphere or to a gaseous fuel storage tank.